The first line conveniently reads from left to right: my %a declares the hash in the current lexical scope in the current thread. : shared shares the hash with all other threads. = (); assigns the empty list to this hash.

The next line has most of its action on the far right side. {}; creates an anonymous hash in the current thread. Due to being anonymous, it's not bound by any scope — it will simply live for as long as something references to it. So {} is kind enough to create and return that reference. That's right: a reference and the value it refers to, are in fact not the same entity. $a{foo} = then stores that reference under the key "foo" in the hash %a.